The invention relates generally to a method for recovering rhenium from a superalloy, e.g., from superalloy scrap. More particularly, the invention relates to a method for recovering rhenium from superalloy scrap by increasing the surface area of the superalloy scrap.
The extensive use of rhenium in fabricating high temperature alloys, its use as a catalyst in a variety of applications, its high cost, and its rarity, are important incentives for developing techniques for its recovery. (Increasing use and growing prices for rhenium are essential factors encouraging the processing of rhenium-containing wastes.) During the manufacture of rhenium containing alloys, a large quantity of machining scrap is produced. Double-component waste, such as molybdenum-rhenium or tungsten-rhenium alloys, can be readily decomposed and processed for recovering rhenium. However, most of the rhenium is contained in superalloys, especially in nickel-based superalloy waste.
Superalloys are high melting, high strength and extremely wear-resistant alloys of a comparatively large number of metals, which are used predominantly in high temperature applications, usually higher than 500 degrees Celsius. In contrast to normal alloys, superalloys are not attacked in oxidizing and corrosive environments. The technological development of superalloys is linked inextricably to gas turbines. The nickel-based superalloys are most suitable for gas turbine construction, especially aircraft turbines.
Recovery of rhenium from superalloy waste is of economic interest because it is a relatively scarce and precious metal. In some special superalloys, rhenium is found in a proportion of up to 6% by weight. However, rhenium is difficult to recycle economically, after its service life.
Most of the prior attempts have focused on pyro- and hydrometallurgical approaches for recovering rhenium from superalloys, where the superalloy is acid-digested, followed by separation of rhenium by salt precipitation or ion-exchange. However, these approaches may not always be economically suitable, because of their cost-intensive complexity, or their time-consuming nature. These processes may also result in large by-product wastes. In contrast, oxidation-based recovery of rhenium from superalloys could be a simple and efficient method. However, as understood by those of ordinary skill in the art, liberation of rhenium from superalloy waste can be an inefficient process, due in part to coarse oxidation feedstock.
It would therefore be desirable to develop new methods for efficiently recovering rhenium from metal alloys—especially superalloy feedstock material.